1. Field of the Invention
The present invention relates to ZSM-5 zeolite catalyst material for use in the synthesis of MTBE and similar ethers. More specifically, the invention relates to ZSM-5 zeolite materials doped with fluorine species and properly activated at high temperatures. The present invention also relates to a method of preparing such catalysts.
2. Description of the Prior Art
MTBE (methyl-tert-butyl ether) is currently used as antiknock additive in gasoline. MTBE is synthesized industrially from methanol and isobutene. In industrial operations this synthesis is performed over an acidic ion-exchange resin [1]. The currently commercially successful acidic ionexchange resin is sold under the brand name AMBERLYST 15.TM. by Aldrich Chemicals and is in fact an organic macrorelicular cation-exchange resin.
Although AMBERLYST 15.TM. brand resin is efficient in terms of MTHE yield, It suffers from several drawbacks including thermal and chemical instability and the production of deleterious byproducts such as oligomers of isobutens including diisobutenes.
ZSM-5 zeolite was proposed as suitable catalyst material for industrial MTBE synthesis [2]. Indeed, ZSM-5 zeolite catalysts are stable and have a shape-selective pore network which suppresses by-product generation during MTBE synthesis [4,8]. By-product generation of oligomers of isobutene such as diisobutenes is deleterious to MTBE production [4].
However, the ZSM-5 zeolite showed inferior activity and MTBE synthesis yields when compared to AMBERLYST .TM. brand resin (under normal conditions of gas phase synthesis of MTBE with temperature of about 70 to 90.degree. C. and methanol/isobutene ratio of about 1:1) [3,4].
To improve MTHE synthesis yields, it was proposed to increase the surface acidity of the zeolite material by coating it with triflic acid. This required that the zeolite pore size was made sufficiently large to accommodate significant amounts of triflic acid (Y-type zeolite) [4-6]. However, this negatively impacted on unique qualities of zeolite material such as its shape selectivity,
It has also been suggested to incorporate fluorine (F) species as electron-withdrawing species to enhance catalyst surface acidity [7]. However, as pointed out later by Nikolopoulos et al [9], "fluoride modification did not result in an MTBE synthesis activity enhancement unless it was coupled with a high temperature activation process". Despite this general statement, the all authors failed in devising a procedure for the preparation of a high performance ZSM-5 catalyst for high yield synthesis of MTRE Indeed, the studies relating to the present invention show that a catalyst activation temperature of 400.degree. C. [ref.7] provided low yields of MTBE synthesis.
Thus, the prior art has so far failed to provide ZSM-5 catalysts capable of effectively competing in the marketplace with resin type catalysts such as AMBERLYST 15.TM. brand resin.
It is therefore an object of the present invention to provide a novel ZSM-5 catalyst and preparation method which results in an increase of the surface acidity of ZSM-5 zeolite catalyst. The novel catalyst must also provide high MTSE production yields without significantly modifying pore characteristics of the ZSM-5 zeolite catalyst in order to preserve its unique shape selectivity and minimize by-product generation.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that this detailed description, while indicating preferred embodiments of the invention, is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.